Industrial gases which are utilized in small to moderate volumes are stored at ambient temperature in pressurized cylinders from which gas is withdrawn as needed. A pressure regulator generally is used to deliver the gas to the consumer at a constant pressure. Gases which have critical temperatures below ambient temperature are stored as supercritical fluids at maximum pressures determined by the design pressure ratings of the cylinders. Examples of these include low-boiling gases such as nitrogen, oxygen, hydrogen, helium, and methane, which are withdrawn from the storage cylinders without phase change. Gases which have critical temperatures above ambient temperature are stored in cylinders as saturated liquids at their respective vapor pressures, and these liquids vaporize as saturated vapor is withdrawn from the cylinders. Common examples of liquefied compressed gases are chlorine, ammonia, and light hydrocarbons such as propane and butane.
Some liquefied compressed gases have vapor pressures at ambient temperature which are not far above atmospheric pressure. Many of these are specialty gases used by the electronics industry in the manufacture of semiconductor and fiber optic devices. Examples of such specialty gases include tungsten hexafluoride, disilane, dichlorosilane, trimethylsilane, hydrogen fluoride, and boron trichloride. These gases must be delivered from storage cylinders to the manufacturing process tools at high purity under carefully-controlled conditions.
Because these specialty gases have low vapor pressures, i.e., vapor pressures which are not far above atmospheric pressure, condensation can occur under certain operating conditions during gas transfer from a storage cylinder to the process tool. This is undesirable because condensation can interfere with the operation of downstream instruments such as flow controllers, can cause pressure fluctuations in the gas delivery system, and in certain cases can result in impurities in the transferred gas caused by leaching of metal components from the delivery system piping.
It is common practice in the industry to heat and insulate the transfer lines to prevent this undesirable condensation. While this practice is effective in preventing undesirable condensation, heat tracing can be a high-maintenance item which is subject to failure at unpredictable times. Such failure can adversely impact the operation of the process tool and affect the efficiency of the overall manufacturing process. Regular monitoring of heat tracing systems can be cost-prohibitive and as a result usually is not practiced. In addition, the need for heat tracing generally minimizes the distance between the gas storage cylinder and the consumer.
It would be desirable to reduce or eliminate the need for heat tracing and insulation of the lines which transfer these specialty gases from storage cylinders to process tools. The invention described below and defined by the claims which follow addresses this need with a method for transferring such specialty gases which can prevent condensation without the use of heat tracing.
The invention relates to a method for delivering a vapor component product to an end user from a storage system which contains the component in coexisting liquid and vapor phases. The method comprises:
(a) withdrawing from the storage system a component stream comprising vapor at a first total pressure and a first temperature;
(b) passing the component stream through a transfer line to a reservoir having an inlet and an outlet, and condensing vapor in the reservoir to provide a liquid component at a second total pressure and a second temperature at the outlet of the reservoir; and
(c) withdrawing the liquid component from the outlet of the reservoir, vaporizing the liquid component to yield the vapor component, and providing the vapor component to the end user.
The coexisting liquid and vapor phases of the component in the storage system may be contained in a storage vessel, and a vapor stream may be withdrawn from the storage vessel while liquid is vaporized therein. The component stream withdrawn from the storage system in (a) may be provided by regulating the pressure of the vapor stream from the storage vessel to provide the component stream at the first total pressure. If desired, the storage vessel may be heated.
The dynamic pressure differential defined by the difference between the first total pressure and the second total pressure may be at least about 1.25 psi. The difference between the first temperature and the second temperature may be at least about 15xc2x0 F. The component stream in the transfer line may be essentially all vapor.
The component may be an essentially pure compound selected from the group consisting of tungsten hexafluoride, disilane, dichlorosilane, trichlorosilane, trimethylsilane, perfluorobutadiene, and chlorine. The total pressure at any point in the transfer line may be less than the vapor pressure of the compound at a temperature equal to the temperature at that point in the transfer line. The second total pressure at the outlet of the reservoir may be greater than the total pressure of the component stream at the inlet of the reservoir.
The invention also relates to a method for controlling the delivery of a vapor component product to an end user from a storage system which contains the component in coexisting liquid and vapor phases, wherein the method comprises:
(a) withdrawing from the storage system a component stream comprising vapor at a first total pressure and a first temperature;
(b) passing the component stream through a transfer line to a reservoir having an inlet and an outlet, and condensing vapor in the reservoir to provide a liquid component at a second total pressure and a second temperature at the outlet of the reservoir;
(c) withdrawing the liquid component from the reservoir, vaporizing the liquid component to yield the vapor component, and providing the vapor component to the end user; and
(d) controlling the first total pressure and the second temperature such that the component stream in the transfer line is essentially all vapor.
The invention also includes a system for delivering a vapor component product to an end user from a storage system which contains the component in coexisting liquid and vapor phases, wherein the system comprises:
(a) storage vessel which contains the component in coexisting liquid and vapor phases, wherein the storage vessel has an outlet for the withdrawal of vapor from the vessel;
(b) a reservoir having an inlet and an outlet;
(c) a transfer line for transferring a component stream comprising vapor from the storage vessel to the inlet of the reservoir;
(d) cooling means for cooling the reservoir and condensing vapor to form a condensed component therein;
(e) piping means for withdrawing the condensed component from the outlet end of the reservoir; and
(f) heating means for vaporizing the condensed component to provide the vapor component to the end user.
The system may further comprise pressure regulating means installed between the storage vessel and the transfer line. The storage vessel may be made of a metal containing at least 99.9 weight % nickel.
The reservoir may be a vertically-oriented cylindrical vessel having an upper end and a lower end, wherein the outlet is at the lower end, and wherein the vessel has a height to diameter ratio of at least about five. This cylindrical vessel may be made of a metal containing at least 99.9 weight % nickel. The condensed component in the reservoir typically forms a liquid column with a height of at least about 12 inches above the outlet of the reservoir.